1. Field of the Invention
The system of the present invention relates to the security of radio telephones. More specifically, the system of the present invention relates to the scrambling of radio transmitted message data transmitted to prevent the accidental or deliberate eavesdropping of voice communications.
2. Art Background
In today's mobile environment, radio telephones, which connect to central office lines via radio waves, are widely used. However, voice communications through radio telephones are by their nature unsecure. A radio telephone that is set to an active channel or frequency, which is already in use by another party using another radio telephone, will hear the voice activity on that channel. Thus, it is quite easy and often common in certain environments to be able to hear other conversations when using the radio telephone. This is not desirable for purposes of privacy. Furthermore, an unsecured system is unacceptable for a business system which consists of radio telephones.
Existing techniques implemented for scrambling voice data to protect against intentional or unintentional "eavesdropping" employ scrambling or encription algorithms which alter all or a majority of the data bits representing the voice data. Attempts at altering a limited number of the voice data bits have not achieved good results and alteration of the least significant data bit is virtually unnoticeable to the listener. Similarly alteration of the two least significant bits may be noticed by the listener but the voice transmission is still understandable or intelligible. Altering the third and fourth least significant bits causes noticeable changes in the voice sounds, but the listener is still able to comprehend the basic communication. Alteration of the high order bits has a significantly larger impact on the voice sound, but the impact only affects the intelligibility of the sound if the original sound contains amplitude code components in that range (that is, low volume speech will be altered but understandable). Altering a combination of these bits will in many cases cause the sound to be unintelligible; however, in many cases the listener will still be able to discern the content of the speech. In addition, the hardware and software required to manipulate the bits and produce effective scrambling is typically too large and computation intensive for a cellular or cordless handset, particularly because the scrambling must be performed for each byte of voice data.